apps: Add meta option

[shivaraj-bh]

This allows specifying metadata info about the flake app. This has been standardised in Nix at NixOS/nix#11297

Here’s an example of its usage:

# Inside perSystem
{
  apps.hello = {
    program = pkgs.hello;
    type = "app";
    meta.description = "I say hello!";
  };
}

[roberth]

I'm supportive of the idea, but the Nix manual states

The only supported attributes are:

• type [...]
• program [...]

I guess this is somewhat open to interpretation, but allowing any attribute whatsoever does not seem to be the intent.
nix flake check is silent about it, but most likely that's just an oversight.

Could you open a PR to nix that documents the description attribute? That way it can become a standard, and everyone benefits from better metadata and discoverability.

As for this PR, I'd be happy to accept a description option when the Nix PR is accepted, but preferably not a freeformType, because that would let typos in these attribute names to go uncaught, and iirc a freeformType can also cause worse errors when "recursive" definitions are involved (e.g. desrcription = "something ${baseNameOf config.command}" would be an infinite recursion because of the typo+freeformType).

[shivaraj-bh]

Could you open a PR to nix that documents the description attribute? That way it can become a standard, and everyone benefits from better metadata and discoverability.

Good idea! Here’s the PR: NixOS/nix#11297

[shivaraj-bh]

@roberth I have made changes based on what was standardised upstream

[roberth]

I was thinking ideally meta could be a submodule with freeform type, but that would cause description to error or have a default like "", neither of which is an improvement, so this is probably the best solution for now.

Related:

• optional options could help out with this NixOS/nixpkgs#334680

[shivaraj-bh]

@roberth Does it look good now?

[srid]

@roberth Is there anything to be done here before merging? This would be useful to implement: juspay/omnix#163

[srid]

In hindsight, should we add a default here? Taking from program, that is, if it is a derivation. But coercedTo seems lossy, so this probably is not possible.

Until then, users basically will have to explicit set it like follows: